1. Field of the Invention
The invention concerns a method of minimising deformation during rapid cooling of flat metallurgical products, such as sheets, strips, flattened portions, wide sections and the like.
2. Discussion of the Background
Many methods and arrangements have been published in the technical literature, as a means of resolving the problem of macroscopic deformation of flat products on cooling, particularly in continuous production lines giving thermal treatment, such as annealing, quenching and the like.
The normal practice in continuous cooling of strips, sheets, wide sections and the like made of metal, is to cool a product at uniform temperature, by sprinkling or spraying it with one or more fluids, in a zone of rectangular shape, where the front for the action is perpendicular to the direction of displacement of the product, which coincides with its length; cooling is generally carried out on both surfaces of the product, with a constant delivery rate per area (expressed as an important volume of fluid of product per unit of area and of time), but it may also be unilateral.
This practice usually leads to serious deformation of the products and the deformation increases when the speed of cooling and/or the width of the product increase and/or if the speed at which the products advance is reduced. Particularly in the case of quenching metals and alloys, the quenching deformations appear immediately downstream of the front for the action of the fluid, within the range of the highest temperatures where the yield strength of the metal is excessively low; the amplitude of the deformations may be such that they impair the homogeneity of cooling and prevent its normal progress in the installation; in particular, degradations in the surface state due to friction may appear.
Reducing these permanent distortions of the products when the cooling process is over entails either a very high speed of advance (and thus cooling zones which are excessively long and excessively expensive) or, in all cases, final smoothing, planing and detensioning operations by mechanical means (rolling mill, planing machine with rollers or working by traction) which increase the cost of production.
In addition detensioning by controlled traction is necessary for products which have to have a very low residual stress level.
One of the most recently recommended means of avoiding distortion on cooling is taught by U.S. Pat. No. 4,270,959 and uses a method of cooling metal strips with blown air, in such a way that the temperature of the lateral zones of the strip is below that of the central zone throughout the cooling process.
However, this process is not suitable for rapid cooling, where a cooling fluid or a mixture of cooling fluids including at least one vaporizable liquid such as water has to be used, both from the metallurgical point of view (inadequate cooling speed) and from the point of view of permanent deformation after cooling.